1. Field of the Invention
Embodiments of the present invention generally relate to satellite navigation receivers and more particularly to GPS receivers having a digital front end.
2. Description of the Related Art
Satellite navigation systems are made up of multiple, specially designed satellites orbiting Earth. These satellites continuously transmit precise, specific radio frequency (RF) signals that are used to provide location information. One widely adopted satellite navigation system is referred to as the Global Positioning System (GPS). By processing the RF signals from four or more GPS satellites, a GPS receiver can determine its current location (longitude, latitude, and altitude) fairly quickly and with a good degree of accuracy.
FIG. 1 shows the general sections of a conventional GPS receiver. An antenna 101 receives the RF signals from the GPS satellites. The RF signals are then amplified, down converted, filtered, and converted into a digital signal by the analog section 102. Specifically, a low-noise amplifier 103 amplifies the weak RF signals. A mixer 104 and local oscillator 105 down converts the amplified RF signals to a lower intermediate frequency (IF) signal. A band pass filter 106 is used to filter out interference. The filtered IF signal is then converted into an equivalent digital IF signal by means of a 1 or 2-bit analog-to-digital converter (ADC) 107. The digital IF signal is then input to a digital section 108. The acquisition engine 109 and tracking engine 110 of the digital section 108 process the digital IF signal to generate acquisition and tracking data which is then input to the processing section 112 by means of register 111. The central processing unit (CPU) 113 analyzes the acquisition and tracking data according to programming instructions stored in the memory 114 and produces the final location information.
This conventional GPS receiver architecture places a great deal of reliance on the performance of the analog section. This is due, in part, because GPS satellite transmitter power may be as low as approximately 22 watts, and the signal travels over 12,000 miles through space and Earth's atmosphere. By the time the GPS signal reaches the GPS receiver, it is extremely weak and degraded. Typically, the received power of a GPS signal is −130 dBm or below. As a result, the analog section 102 must amplify, filter, and perform significant signal processing on the weak, attenuated analog GPS signal before it can be converted into a digital signal by the ADC 107.
Further complicating matters is that the GPS signal encounters a variety of interferences. Unfortunately, the GPS band falls within a crowded frequency spectrum, with strong RF signals that are only a few tens of MHz on either side of the protected GPS band. In addition, RF leakage and harmonics of the digital clock in the GPS receiver may appear to be very close or even fall within the GPS band. And if interference signal(s) are present at the input to the ADC 107, then some of the ADC's dynamic range is allocated to accommodate for this interference in order to avoid severe clipping. As a consequence, the desired GPS signal is sized smaller, the quantization noise relative to the thermal noise increases, and the overall performance of the GPS receiver degrades. To account for these and other deleterious factors, the design of the analog section must meet extremely stringent and exacting specifications. Consequently, the analog section occupies a relatively large area of circuitry, consumes a great amount of power, and is expensive to implement.
Therefore, the analog section is fast becoming a major impediment to the introduction of smaller, cheaper, lighter, more accurate, longer lasting portable battery-operated GPS receivers that markets and consumers crave.